1. Field of the Invention
This document relates to a liquid crystal display and a method of controlling dot inversion thereof.
2. Discussion of the Related Art
An active matrix type liquid crystal display displays moving images using thin film transistors (hereinafter, referred to as “TFTs”) as switching elements. In comparison with a cathode ray tube (CRT), the liquid crystal display can have a smaller size. Thus, the liquid crystal display is used as displays in portable information devices, office equipment, computers, televisions, etc., and hence is fast replacing the cathode ray tube.
Liquid crystal cells of the liquid crystal display display a picture image by changing transmittance by a potential difference between a data voltage supplied to a pixel electrode and a common voltage supplied to a common electrode. The liquid crystal display is generally driven by an inversion scheme of periodically inverting the polarity of the data voltage applied to the liquid crystal cell in order to prevent deterioration of the liquid crystal. When the liquid crystal display is driven by an inversion scheme, the liquid crystal display may have a low picture quality according to a correlation between the polarities of data voltages charged in the liquid crystal cells and a data pattern of an input image. This is because the polarity of data voltages charged in the liquid crystal cells are not balanced between the positive and negative polarities but either of the positive and negative polarities becomes dominant, and hence the common voltage applied to the common electrode is shifted. Once the common voltage is shifted, the reference potential of the liquid crystal cells is shifted, and this causes a viewer to feel flicker or smear on an image displayed on the liquid crystal display.
FIGS. 1 and 2 show data examples of problem patterns which may cause degradation of picture quality when driving a liquid crystal display by dot inversion.
Among the problem patterns, a pattern, as shown in FIG. 1, in which (white) pixel data having a white gray scale and (black) pixel data having a black gray scale alternate in units of one pixel is referred to as a shutdown pattern. Each pixel data comprises red subpixel data (R), green subpixel data (G), and blue subpixel data (B). As for a shutdown pattern detection method, shutdown patterns included in an input image are counted and whether the input image is data of a shutdown pattern or not is determined in accordance with the count value. In the shutdown pattern detection method, for example, if N-th (N is a positive integer) pixel data is white gray scale pixel data and (N+1)-th pixel data is black gray scale pixel data, the count value of a problem pixel counter is increased by 1 at a time, and the data of the input image is judged as having a shutdown pattern when the count value is above a predetermined threshold value.
Among the problem patterns, a pattern, as shown in FIG. 2, in which (white) pixel data having a white gray scale and (black) pixel data having a black gray scale alternate in units of two pixels is referred to as a smear pattern. As for a smear pattern detection method, similarly to the shutdown pattern method, smear patterns included in an input image are counted and whether the input image is data of a smear pattern or not is determined in accordance with the count value. In the smear pattern detection method, for example, if N-th pixel data and (N+1)-th pixel data are white gray scale pixel data and (N+2)-th pixel data and (N+3)-th pixel data are black gray scale pixel data, the count value of the problem pixel counter is increased by 1 at a time, and the data of the input image is judged as having a smear pattern when the count value is above a predetermined threshold value.
The problem patterns include various types of patterns that cause degradation of picture quality in dot inversion, as well as the shutdown pattern and the smear pattern. One of these patterns is a flicker pattern as shown in FIG. 14. In the flicker pattern, white gray scale subpixel data and black gray scale subpixel data alternate up and down and left and right.
However, a method of detecting a problem pattern from an input image involves storing a large amount of problem pattern data in advance for each problem pattern, and a large number of detection logic modules are required to detect each of the problem pattern data. For instance, in order to recognize a shutdown pattern, it is necessary to define, in advance, a maximum of (23−1)×2=14 patterns as shown in FIG. 3 that may appear in six subpixels, and a detection logic module for detecting each of the patterns is required. In case of the smear pattern, it is necessary to define, in advance, a maximum of (26−1)×2=126 patterns that may appear in 12 subpixel data, and a detection logic module for detecting each of the patterns is required.